Abstract:
Following the core conversion of Ghana’s Miniature Neutron Source Reactor (MNSR) from high enriched uranium (HEU) to low enriched uranium (LEU), there has been a change in the fuel composition, fuel, clad and other reactor core parameters. Since, the allowable core power in a nuclear reactor is limited by thermal considerations, this study presents thermal hydraulic analysis of the LEU core of the Ghana research reactor 1(GHARR-1). Neutronic parameters of GHARR -1 with LEU core was modelled and simulated using the Monte Carlo N Particle code (MCNP), the core was modelled into two channels, the hottest pin and the average of the remaining pins. Axial and Radial power peaking factors were computed, as well as reactivity coefficients for transient and steady state analysis. The Program for reactor transients (PARET/ANL) was used to model the core under transient conditions of various reactivity insertions. For lower reactivity insertions of 1.94 mk and 2.1 mk, the reactor powers peaked to 30.2 kW and 34.2 kW. And the coolant temperatures were 52.42 ◦C and 54.24 ◦C respectively, far below the saturation temperature of 100 ◦C at a pressure of 1 atm, hence boiling is not expected to occur. The peak clad and fuel temperatures were found to be 62.73◦C and 63.24 ◦C for 1.94 mk and 65.53 ◦C and 66.10 ◦C for 2.1 mk respectively. These clad and fuel temperatures are far below the melting points of the Zicaloy-4 clad material and UO2 fuel. Accidental insertion of large reactivity of 6.71 mk was simulated and studied, the coolant temperature was found to be 98.8 ◦C, which is close to the coolant saturation temperature, but boiling of the coolant is not envisaged at such large reactivity insertions. But the clad material and the fuel material will be uncompromised as the peak temperatures determined at such large reactivity insertions were lower than their melting points. The plate temperature code (PLTEMP/ANL) was used to model steady state operation of the core. Safety margins; Onset of Nucleate Boiling Ratio (ONBR), Departure from Nucleate Boiling Ratio (DNBR), and Flow Instability Ratio (FIR) were found to be within the safety margins and hence no boiling crisis will occur in the core. Also there is efficient transfer of heat from the fuel to clad. The temperature drop from fuel meat to clad was found to be very low in the range of 0.50 ◦C to 3.18 ◦C for transients and 0.16 ◦C to 0.34 ◦C under steady state conditions. The results obtained were found to agree with the available experimental results. Similar trends were observed when HEU and LEU results were compared.
